Cationic polymerizable compositions and methods of use thereof

ABSTRACT

An inkjet printing method and inkjet compositions are disclosed. The method includes selectively depositing by inkjet printing, layer by layer, a first composition and a second composition onto a receiving media from different dispensers to form polymerizable deposited layers. The first composition includes one or more free-radical polymerizable compounds and a cationic photoinitiator and is devoid of compounds able to undergo cationic photopolymerization within the first composition. The second composition includes one or more cationic polymerizable compounds and is devoid of cationic photoinitiators. At least one of the compositions includes a radical photoinitiator. The method further includes exposing the deposited layers to actinic radiation to initiate polymerization of the free-radical polymerizable compounds and the cationic polymerizable compounds within the deposited layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.14/381,628, filed Aug. 28, 2014, which is a National Phase Applicationof PCT International Application No. PCT/IL2013/050176, InternationalFiling Date Feb. 28, 2013, claiming priority of U.S. Provisional PatentApplication No. 61/605,467, filed Mar. 1, 2012, all of which are herebyincorporated by reference.

BACKGROUND

Three-dimensional (3D) inkjet printing is a known process for buildingthree dimensional objects by selectively jetting building materials, forexample, photo polymerizable compositions, via ink-jet printing headnozzles onto a printing tray in consecutive layers, according topre-determined image data. Actinic radiation, for example, ultraviolet(UV) radiation is directed onto the deposited layers ofphotopolymerizable compositions to solidify or stabilize the layers.

A drawback of 3D printing of UV curable compositions is the tendency ofthe liquid formulation to solidify on the printing head nozzle plateduring the jetting process due to UV reflections, heat or both. Nozzleplate contamination is more severe with compositions containingcationically polymerizable components since cationic polymerization isnot inhibited by oxygen and is accelerated by heat. Additionally, oncethe cationic polymerization mechanism has been initiated, it continueseven when not exposed to light. The use of cationic polymerizablecompositions is desirable, however, due to certain valuable properties,such as, for example, relatively low shrinkage, high thermal resistanceand high chemical and solvent resistance of such compositions.

SUMMARY

Features of certain embodiments may be combined with features of otherembodiments; thus certain embodiments may be combinations of features ofmultiple embodiments.

There is thus provided according to embodiments of the invention aninkjet printing method. The method includes selectively depositing byinkjet printing, layer by layer, a first composition and a secondcomposition onto a receiving media from different dispensers to formpolymerizable deposited layers, wherein the first composition comprisesone or more free-radical polymerizable compounds and a cationicphotoinitiator and is devoid of compounds able to undergo cationicphotopolymerization within the first composition, and the secondcomposition comprises one or more cationic polymerizable compounds andis devoid of cationic photoinitiators, and at least one of the first andthe second compositions comprises a radical photoinitiator; and furtherincludes exposing the deposited layers to actinic radiation to initiatepolymerization of the one or more free-radical polymerizable compoundsand the one or more cationic polymerizable compounds within thedeposited layers.

According to embodiments of the invention, the first and secondcompositions are dispensed so as to form a substantially isotropicdistribution within each of the deposited layers. According toembodiments of the invention, the first and second compositions aredispensed such that in each of the deposited layers, a ratio betweendeposited droplets of the first composition and deposited droplets ofthe second composition is between about 25% first composition to about75% second composition and about 75% first composition to about 25%second composition. According to embodiments of the invention, the ratioin at least one layer of the deposited layers is different to the ratioin at least one other of the deposited layers.

There is thus provided according to embodiments of the invention aprinting composition. The printing composition comprises a free radicalcomposition having one or more free-radical polymerizable compounds anda cationic photoinitiator and is not polymerizable by a cationicpolymerization mechanism; and a cationic composition having one or morecationic polymerizable compounds and devoid of cationic photoinitiators.At least one of the free radical and the cationic compositions comprisesa radical photoinitiator, and the free radical composition and thecationic composition are kept separate from each other prior to andduring dispensing. In some embodiments, the free radical compositioncomprises at least one acrylic monofunctional monomer at a concentrationof at least 20% by weight relative to the weight of the free radicalcomposition and an acrylic oligomer at a concentration of at least 20%by weight relative to the weight of the free radical composition.According to some embodiments, the cationic composition comprises3,4-Epoxy cyclohexyl methyl-3,4 epoxy cyclohexyl carboxylate at aconcentration of at least 50% by weight relative to the weight of thecationic composition. In some embodiments, the 3,4-Epoxy cyclohexylmethyl-3,4 epoxy cyclohexyl carboxylate constitutes above 90% by weightof the cationic composition. In some embodiments, the cationicphoto-initiator comprises Aryliodunium Hexaflouoroantimonate. In someembodiments the cationic composition further comprises acrylicpolymerizable compounds. According to some embodiments, the cationicpolymerizable compound is selected from cycloaliphatic epoxides, vinylethers, cyclic sulphides, lactones and siloxanes. In some embodiments,the free radical composition further comprises one or more hydroxylcontaining compounds, which do not polymerize in the absence of acationic photopolymerizable compound.

There is further provided according to embodiments of the invention aprinting material kit for inkjet printing. The kit comprises a firstcontainer storing a first composition, the first composition comprisingone or more free-radical polymerizable compound, and a cationicphotoinitiator, the first composition not being polymerizable by acationic reaction mechanism; and a second container storing a secondcomposition, the second composition comprising one or more cationicpolymerizable compounds and devoid of cationic photoinitiators.

In some embodiments, the first composition of the kit comprises at leastone acrylic monofunctional monomer at concentration of at least 20% byweight relative to the weight of the first composition and at least oneacrylic oligomer at a concentration of at least 20% by weight relativeto the weight of the first composition. In some embodiments, the secondcomposition of the kit comprises a di-functional silicon containingresin. In some embodiments, the second composition of the kit comprisesa cationic photo-sensitizer.

There is further provided according to embodiments of the invention athree-dimensional object. The three-dimensional object comprises amultiplicity of polymerized deposited layers formed by selectivelydepositing by inkjet printing, layer by layer, a first composition and asecond composition onto a receiving media from different dispensers toform polymerizable deposited layers, wherein the first compositioncomprises one or more free-radical polymerizable compounds and acationic photoinitiator and is not polymerizable by a cationicmechanism, and the second composition comprises one or more cationicpolymerizable compounds and is devoid of cationic photoinitiators, andat least one of the first and the second compositions comprises aradical photoinitiator, and exposing the polymerizable deposited layersto actinic radiation to initiate polymerization of the one or morefree-radical polymerizable compounds and the one or more cationicpolymerizable compounds.

There is further provided according to embodiments of the invention asolid composition formed by selectively depositing, layer by layer, thecompositions detailed above so as to form deposited layers, each havinga pattern with a substantially isotropic distribution of the freeradical composition and the cationic composition and exposing thedeposited layers to actinic radiation to initiate polymerization of theone or more free-radical polymerizable compounds and the one or morecationic polymerizable compounds within the deposited layers. in someembodiments, The solid composition exhibits a heat deflectiontemperature (HDT) of above 100C°.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However it will be understood by those of ordinary skill in the art thatthe embodiments of the present invention may be practiced without thesespecific details. In other instances, well-known methods, procedures andcomponents have not been described in detail so as not to obscure thepresent invention.

Embodiments of the invention are directed to an inkjet printing methodand compositions that replace an inkjet composition containing bothacrylic and epoxide polymerizable components. While the acrylic-basedcomponent produces negligible contamination of the nozzle plate due tooxygen inhibition, the epoxy-based component tends to polymerize on thenozzle plate within a few hours of printing, leading to rapidcontamination of the printing head. According to embodiments of theinvention instead of using a single composition, two compositions, eachdeposited from a different printing head are used. Both compositions aresubstantially “non-reactive” when passing through the inkjet printinghead and nozzle plate, and therefore the inkjet heads are not clogged bypolymerization during the printing process.

Embodiments of the present invention are directed to photocurable orpolymerizable compositions suitable for inkjet printing that do not clogthe nozzle plate of the printing heads. A first composition may compriseone or more free-radical polymerizable compounds or components which donot polymerize by cationic mechanism, and a cationic photoinitiator, andbe devoid of compounds able to undergo cationic polymerization withinthe first composition. A second composition may contain one or morecationic polymerizable compounds or components and be devoid of orsubstantially free of cationic photoinitiators. The cationic compositionmay optionally include acrylic monomers. The compositions are depositedfrom separate printing heads, to be digitally combined or mixed afterprinting within each deposited layer.

The first composition may further comprise hydroxyl containingcompounds, which do not polymerize in the absence of a cationicphotopolymerizable compound such as epoxy.

Both the first and the second compositions or only one of them mayfurther include a free radical polymerization photoinitiator. Thecompositions may further include additional substances such asinhibitors, photosensitizers, surfactants, toughening agents, pigments,dyes, fillers and others.

The first composition, namely the radical composition may include acombination of acrylic monomers and oligomers with a degree of purityhigh enough not to inhibit cationic polymerization. It has been foundthat alkaline and/or sulfur impurities significantly inhibit thecationic polymerization reaction.

For ease of explanation, the first composition may be referred to as a“radical composition” and the second composition may be referred to as a“cationic composition”. In some embodiments, the compositions are foruse in two dimensional inkjet printing and in other embodiments, thecompositions are for use in three dimensional (3D) inkjet printing.These two compositions may be regarded as a two-part composition forinkjet printing, referred to herein as a printing composition, speciallydesigned for a method including selectively depositing each of the firstand second compositions onto a receiving media, e.g. a printing tray,from different dispensers to form a deposited layer comprising dropletsof both compositions and exposing the deposited layer to actinicradiation to initiate polymerization or partial polymerization of theone or more free-radical polymerizable components of the firstcomposition and the one or more cationic polymerizable components of thesecond composition with the layer.

Accordingly, embodiments of the present invention are directed to amethod of forming a three dimensional object by inkjet printing. Themethod includes selectively depositing, layer by layer, a firstcomposition and a second composition onto a receiving media, e.g.printing tray, from two different dispensers to form polymerizabledeposited layers, wherein the first composition comprises one or morefree-radical polymerizable compounds which do not polymerize by cationicmechanism and a cationic photoinitiator, and is devoid of compounds ableto undergo cationic polymerization within the first composition, and thesecond composition comprises one or more cationic polymerizablecompounds and is devoid of cationic photoinitiators, and at least one ofthe first and the second compositions comprises a radicalphotoinitiator, and exposing the deposited layers to actinic radiationto initiate polymerization of the one or more free-radical polymerizablecompounds and the one or more cationic polymerizable compounds withinthe deposited layers. The method may further include a post-printingheating operation to strengthen the final object.

According to embodiments of the invention, the first and the secondcompositions are deposited in patterns that enable the cationicphotoinitiator in the first composition to diffuse into the secondcomposition and in such a way enabling the polymerizable components ofthe second composition to polymerize or at least partially polymerizeupon exposure to radiation, such as UV radiation. For example, an Objet®Connex500™ 3D printing system of Stratasys Ltd. allows separate jettingof two or more materials in a single layer. Other printing systems maybe used.

In some embodiments, a first composition of the printing composition mayinclude one or more free-radical polymerizable compounds or componentswhich do not polymerize by cationic mechanism and a cationicphotoinitiator and be devoid of or substantially free of cationiccompounds or compounds able to undergo cationic polymerization withinthe first composition and is further substantially free of a radicalphotoinitiator, and a second composition includes one or more cationicpolymerizable compounds or components and a radical photoinitiator andis devoid of or substantially free of cationic photoinitiators. In suchembodiments, the first and the second compositions are deposited in apattern that enables the radical photoinitiator in the secondcomposition to diffuse into the first composition and the cationicphotoinitiator in the first composition to diffuse into the secondcomposition. In such a way, the polymerizable components of the firstand second compositions polymerize or at least partially polymerize onthe printing tray upon exposure to UV radiation or other actinicradiation, to form a layer of solid or semi-solid material.

By separating the cationic photoinitiator and the cationic polymerizablecomponents, and optionally the radical photoinitiator and radicalpolymerizable components, the jetted cationic and radical compositionswill be substantially non-reactive before and during jetting, and thusindirect radiation originating from reflections from the printing trayor other mechanical parts will not polymerize these non-reactivecompositions on the printing head nozzle plate. It has been found thatthe rate of formation of a film of polymerized radical component on thenozzle plate due to radiation reflections is significantly lower thanthe rate of formation of a film of polymerized cationic components onthe nozzle plate. The lower rate of film formation for a radicalcomponent may be due to the radical polymerization mechanism, which isnaturally inhibited by oxygen. Accordingly, in some embodiments, thefirst composition may include both free-radical polymerizable compoundsor components and a radical photoinitiator and be reactive duringjetting without interfering with the printing process.

Embodiments of the invention are further directed to a printing materialkit suitable for inkjet printing. The kit may include a first containerstoring a first composition (the radical composition) and a secondcontainer storing a second composition (the cationic composition),wherein the first composition includes one or more free-radicalpolymerizable compounds or components which do not polymerize bycationic mechanism and a cationic photoinitiator and is free of cationicpolymerizable compounds or components able to undergo cationicpolymerization within the first composition, and optionally free of aradical photoinitiator; and the second composition includes one or morecationic polymerizable compounds or components and optionally a radicalphotoinitiator and is free of cationic photoinitiators. Both the radicaland the cationic compositions or only one of them may further include afree radical polymerization photoinitiator.

The compositions may exhibit the following characteristics to operatesatisfactorily within an inkjet printing system: viscosity values in therange of about 10-25 centipoise at a jetting temperature, usually in therange of 25° C.−120° C. to allow jetting through the inkjet nozzles;surface tension of about 26 to 32 Dyne/cm and Newtonian liquid behaviorto allow continuous jetting of the liquid compositions; and reactivityto provide rapid polymerization of deposited thin layers of materialwhen exposed to UV irradiation. In some embodiments, the first andsecond compositions are used in 3D printing, to produce a combinedpolymerized material with a heat distortion temperature (HDT) of above100° C. In some embodiments, the polymerized material resulting fromjetting of the first and the second compositions may exhibit an HDT ofabove 120° C. or above 140° C. Both the first and the secondcompositions may be prepared in any suitable manner, for example bymixing all the composition components together or by pre-mixing certaincomponents and then mixing them with the remaining components.

Embodiments of the invention are directed to an inkjet printing methodusing the first and second compositions detailed herein. The methodincludes selectively depositing each of the compositions onto areceiving media, e.g. a printing tray, from different dispensers to forma layer and exposing the thus deposited layer to actinic radiation toinitiate polymerization or at least partial polymerization of the one ormore free-radical polymerizable components of the first composition andthe one or more cationic polymerizable components of the secondcomposition.

In some embodiments, the method may include building a three dimensionalarticle by selectively dispensing, layer by layer, the first and secondcompositions, each from a different dispenser or printing head to formlayers, corresponding to cross-sectional layers of a three dimensionalarticle. According to some embodiments, each of the first and secondcompositions may be deposited in pre-designed patterns. The patterns mayinclude a multi-layer pattern such that the image of a first layer maybe designed to be complementary to the image of a subsequent layer. Thepattern may be designed to enable maximal homogeneity between the firstand second compositions within a layer and/or within two or moreconsecutive layers. Additionally or alternatively, the pattern may bedesigned such that there would be maximal contact between the first andsecond compositions. The pattern may include a “zigzag” pattern, anysuitable interlaced pattern or a Chess board pattern. A differentpattern may be created in at least two different layers. The interlacedpatterns may overlap or be non-overlapping between successive layers.

The digital mode of printing, namely, deposition of the relative amountsof different materials (ratio between the first and second compositions)and their positioning within a layer according to the image data (i.e.layer pattern) can determine the physical and thermo-mechanicalproperties of the resulting polymerized material, for example, strength,shrinkage and thermal resistance. A desired property of the resultingthree dimensional object is exhibiting minimal curling deformation. Thecurling deformation is the tendency of the edges of a base of theprinted three dimensional object to lift off the printing tray. Thecurling may be quantified by printing for example a 10×10×230 mm bar forcurling measurement (deformation bar) and measuring the gap between theprinting tray/printing surface/other flat surface and the bottom surfaceof the printed deformation bar. Measurement may be done for example by acaliper measurement ruler, such as a ruler distributed by AbsoluteDigimatic Caliper, Mitutoyo, USA.

It has been surprisingly found that not only the ratio between theamount of the different materials forming each object layer is importantin order to produce a part with desired properties. The manner or orderin which the materials (first and second compositions) are combined isalso important. It has been found that optimal results with respect tocurling are obtained when mixing of the compositions is done in eachlayer and in a homogeneous, substantially isotropic way. It has beensurprisingly found that minimal curling of the object being printed isachieved when depositing the acrylic composition (first composition) andthe epoxy composition (second composition) in such a pattern that thecombining of the two compositions occurs within each layer, layer bylayer, and distribution of the two compositions within each layer ishomogeneous and isotropic. With such a homogeneous and isotropic patternthe curling deformation may be around 1-2 mm.

The ratio between first and second compositions within a given layer maynot necessarily be the same ratio as in the article or object as awhole. For example, a first layer may contain a larger quantity of thefirst composition and the second subsequent layer may contain a smallerportion of the first composition. The ratio between the first (radical)and second (cationic) compositions within the article may vary betweenabout 25% to 75% by % droplets or dots of the total droplets of a layer(first composition) to about 75% to 25% by dots (second composition).For example, the digital print mode may include depositing the first andsecond compositions within a layer in equal quantities in a Chess boardpattern (i.e. 50% by dots of the first composition (radical composition)and 50% by dots of the second composition (cationic composition). Thechess board pattern may include a pattern of alternating areas whereeach area may represent a pixel, a square 2×2 pixels, a square of 4×4pixels and any other area size. In another embodiment, the digital printmode may include depositing within a layer about 40% (by dots) radicalcomposition and about 60% cationic composition, or vice versa.

In some embodiments, at least one layer of the 3D object being printedmay be comprised of 100% first composition and 0% second composition. Agiven layer may be comprised of 25-100% first material and 0-75% secondmaterial.

It has been found that in certain cases, increasing the cationic portionwithin a deposited layer results in an increase of the HDT value of thecombined polymerized material, while decreasing the cationic portion maylead to a decrease in the HDT value of the combined polymerizedmaterial. For instance, a digital mode that is based on about 60%cationic composition and about 40% radical composition may result in aphoto-polymerized material having an HDT of about 140° C. and a digitalmode that is based on about 50% cationic composition and about 50%radical composition may result in a photo-polymerized material having anHDT of about 125° C. Furthermore, it has been found that additionalreduction in cationic composition ratio to about 25% cationiccomposition and about 75% respectively may decrease the HDT of theresulting photo-polymerized material to 75° C.

In some embodiments, the ratio between the first and the secondcomposition is different in different layers. For example, the ratio maybe different in consecutive layers or in different regions of thearticle. In some embodiments, the ratio between the first and secondmaterial may change gradually between successive layers or regions.

Free-Radical Polymerizable Components

The first composition or radical composition according to embodiments ofthe invention contains one or more free-radical polymerizable componentsor compounds which do not polymerize by cationic mechanism. Thefree-radical polymerizable component may include as the polymerizablereactive functional groups, a (meth)acrylic functional group. The term“(meth)acrylic” or “(meth)acrylate” refers to both acrylates andmethacrylates. Free-radical polymerizable components may include(meth)acrylic monomers, (meth)acrylic oligomers, and any combinationthereof. Other free-radical polymerizable compounds may include thiols,vinyl ethers and other reactive double bonds.

An acrylic oligomer is a functional acrylated molecule which may be, forexample, polyesters of acrylic acid and methacrylic acid. Other examplesof acrylic oligomers are the classes of urethane acrylates and urethanemethacrylates. Urethane-acrylates are manufactured from aliphatic oraromatic or cycloaliphatic diisocyanates or polyisocyanates andhydroxyl-containing acrylic acid esters. Oligomers may bemono-functional or multifunctional (for example, di-, tri-,tetra-functional, and others). An example is a urethane-acrylateoligomer marketed by IGM Resins BV (The Netherlands) under the tradename Photomer-6010.

An acrylic monomer is a functional acrylated molecule which may be, forexample, esters of acrylic acid and methacrylic acid. Momoners may bemono-functional or multifunctional (for example, di-, tri-,tetra-functional, and others). An example of an acrylic mono-functionalmonomer for the present invention is phenoxyethyl acrylate, marketed bySartomer Company (USA) under the trade name SR-339. An example of anacrylic di-functional monomer is propoxylated (2) neopentyl glycoldiacrylate, marketed by Sartomer Company (USA) under the trade nameSR-9003.

Either the monomer or the oligomer might be polyfunctional, which aremolecules which may provide enhanced crosslinking density. Examples ofsuch molecules are Ditrimethylolpropane Tetra-acrylate (DiTMPTTA),Pentaerythitol Tetra-acrylate (TETTA), Dipentaerythitol Penta-acrylate(DiPEP).

A first or radical composition may comprise one or more free radicalpolymerizable components and a cationic photoinitiator, and be devoid ofor substantially free of cationic polymerizable compounds or able toundergo cationic polymerization within the first composition. Thefree-radical polymerizable component(s) may be present in the first,i.e. radical composition in a concentration from about 50% to about 99%by weight of the total first composition. In some embodiments,free-radical polymerizable components may be present in the firstcomposition in an amount above 85% by weight of the total firstcomposition. An exemplary radical composition may include Epoxyacrylateoligomer (PH3016) in concentration of from about 30% to about 50% byweight of the total radical composition (e.g. about 50%),Phenoxya-crylate (SR339) in a concentration of from about 10% to about70% by weight of the total radical composition (e.g. about 50%).

The first composition may further comprise hydroxyl containingcomponents, which do not polymerize in the absence of a cationicphotopolymerizable component such as epoxy.

Cationic Polymerizable Compounds

The second or cationic composition according to embodiments of theinvention contains one or more cationically and optionally alsonon-cationically polymerizable components. The cationicallypolymerizable components may include epoxy, caprolactam, caprolactone,oxetane, vinyl ether. Non-limiting examples of epoxy compounds includeBis-(3,4 cyclohexylmethyl) adipate, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexyl carboxylate, 1,2epoxy-4-vinylcyclohexane, 1,2-epoxy hexadecane and 3,4-epoxycyclohexylmethyl-3,4-epoxy cyclohexane carboxylate, which is available,for example, under the trade name UVACURE 1500 from Cytec SurfaceSpecialties SA/NV (Belgium) and mono or multifunctional silicon epoxyresins such as PC 1000 which is available from Polyset Company (USA).

The cationic polymerizable compound(s) may be present in the secondcomposition in concentration from about 10% to about 100% by weight ofthe total second composition. In some embodiments, the cationicpolymerizable compound(s) may be present in the second composition in aconcentration from about 50% to about 95% by weight of the total secondcomposition. In a further embodiment the cationic polymerizablecomponents in the second composition may be present in a concentrationof about 85% to about 95% by weight of the total second composition.

Free Radical Polymerization Initiators

The free radical polymerization photoinitiator or in short the freeradical photoinitiator may be a UV free radical initiator that willstart a free radical polymerization reaction when exposed to radiationin the UV light spectrum. The photoinitiator may be a single compound ora combination of two or more compounds, which form an initiating system.Each of the first and second compositions of both may include one ormore radical photo-initiators. The radical photo-initiator of the firstcomposition and of the second composition may be the same or different.

The radical photo-initiator may be a compound that produces a freeradical on exposure to radiation such as ultraviolet or visibleradiation and thereby initiates a polymerization reaction. Non-limitingexamples of some suitable photo-initiators include benzophenones(aromatic ketones) such as benzophenone, methyl benzophenone, Michler'sketone and xanthones; acylphosphine oxide type photo-initiators such as2,4,6-trimethylbenzolydiphenyl phosphine oxide (TMPO),2,4,6-trimethylbenzoylethoxyphenyl phosphine oxide (TEPO), andbisacylphosphine oxides (BAPO's); benzoins and bezoin alkyl ethers suchas benzoin, benzoin methyl ether and benzoin isopropyl ether and thelike. Examples of photo-initiators are bisacylphosphine oxide (BAPO's),marketed by Ciba under the trade name 1-819.

The free-radical photo-initiator may be used alone or in combinationwith a co-initiator. Co-initiators are used with initiators that need asecond molecule to produce a radical that is active in the UV-systems.Benzophenone is an example of a photoinitiator that requires a secondmolecule, such as an amine, to produce a curable radical. Afterabsorbing radiation, benzophenone reacts with a ternary amine byhydrogen abstraction, to generate an alpha-amino radical which initiatespolymerization of acrylates.

Non-limiting examples of suitable UV free radical initiators are2,2-dimethoxy-2-phenylacetophenone which is available under the tradename IRGACURE 1-651 from Ciba Specialty Chemicals (Switzerland), and2,2-Diethoxyacetophenone available under the trade name Genocure [DEAP]from Rahn AB/Gmbh (Switzerland/Germany). Alpha-hydroxy ketones includingDarocure 1173, Irgacure 184, Irgacure 2959 available from Ciba SpecialtyChemicals (Switzerland) can be also used. Acyl phosphines includingDarocur TPO and Irgacure 819 (Ciba Specialty Chemicals (Switzerland))are less preferable. Alternatively, the free radical initiator may be aninitiator that will start a free radical reaction when exposed toradiation at any suitable wavelength, such as visible light.

The free radical initiator may be present in the first composition in aconcentration from about 0.5% to about 5% by weight of the total firstcomposition. In some embodiments, the free radical initiator may bepresent in the first composition in an amount from about 1% to about 3%by weight of the total first composition. In some embodiments, the freeradical initiator may be additionally present in the second compositionthat contains the cationic polymerizable substance in an amount fromabout 0.5% to about 5% by weight of the total second composition. Insome embodiments, the free radical initiator may be present in thesecond composition in an amount from about 1% to about 3% by weight ofthe total second composition.

Cationic Polymerization Initiators

Suitable cationic photo-initiators for the present invention includecompounds which form aprotic acids or Bronstead acids upon exposure toultraviolet and/or visible light sufficient to initiate polymerization.The photo-initiator used may be a single compound, a mixture of two ormore active compounds, or a combination of two or more differentcompounds, i.e. co-initiators. Non-limiting examples of suitablecationic photo-initiators are aryldiazonium salts, diaryliodonium salts,triarylsulfonium salts, triarylselenonium salts and the like. In someembodiments, a cationic photo-initiator may be a mixture oftriarylsulfonium hexafluoroantimonate salts marketed by Lambson UVI6976. The cationic polymerization phoinitiator or in short the cationicphotoinitiator may be a UV cationic polymerization initiator that willstart a cationic polymerization reaction when exposed to radiation inthe UV light spectrum. Suitable photoinitiators include, for example,those compounds which form aprotic BrØnsted acids upon exposure to UVlight. The photoinitiator may be a single compound or a combination oftwo or more compounds, which form an initiating system.

Onium salts constitute a useful class of cationic photoinitiators.Triaryl Sulfonium (TAS) or diaryliodonium salts are importantrepresentatives of this class of photoinitiators. A non-limiting exampleof a suitable initiator is P-(octyloxyphenyl) phenyliodoniumhexafluoroantimonate available under the trade name UVACURE 1600 fromCytec Company (USA). Alternatively, the cationic initiator may be aninitiator that will start a cationic reaction when exposed to radiationat any suitable wavelength, such as visible light. Non-limiting examplesof cationic initiators are lodonium,(4-methylphenyl)(4-(2-methylpropyl)phenyl)-hexafluorophosphate known asIrgacure 250 or Irgacure 270 available from Ciba Speciality Chemicals(Switzerland), Mixed arylsulfonium hexafluoroantimonate salts known asUVI 6976 and 6992 available from Lambson Fine Chemicals (England),Diaryliodonium Hexafluoroantimonate known as PC 2506 available fromPolyset Company (USA), (Tolylcumyl) iodonium tetrakis(pentafluorophenyl) borate known as Rhodorsil® Photoinitiator 2074available from Bluestar Silicones (USA), Iodoniumbis(4-dodecylphenyl)-(OC-6-11)-hexafluoroantimonate known as Tego PC1466 from Evonik Industries AG (Germany).

The cationic photoinitiator may be present in the first composition in aconcentration from about 0.2% to about 7% based on the total weight ofthe first composition. In some embodiments, the cationic initiator maybe present in the first composition from about 2% to about 4% by weightof the total first composition.

Cationic Photosensitizers

The efficiency of the photopolymerization process is strongly dependenton the overlap between the emission spectrum of the UV light source andthe absorption spectrum of the photoinitiator. As the emission spectrumof normally used medium pressure mercury lamp does not necessarilyoptimally fit the excitation peak of iodonium salts, a sensitizer havingan absorption spectrum different to that of the photoinitiator may beadded to the composition. A variety of compounds can be used asphotosensitizers in a cationic system including, for example,heterocyclic and fused ring-aromatic hydrocarbons. Non-limiting examplesof cationic photosensitizers include dibutoxyanthrancene, phenothiazine,anthracene, curcumin and 2-isopropyl thioxanthone.

Toughening Agents for the Cationic Composition

Epoxy resins which are polymerized in a cationic mechanism might bebrittle and notch sensitive. Thus, toughening agents may be added to thecationic composition. Toughness modification of epoxy resins may becarried out in different ways including incorporation of an elastomericdispersed phase and/or incorporation of components undergoing phaseseparation during curing. Non-limiting examples of a toughening agentinclude epoxidized polybutadiene (PB3600 (Daicel Corp., Japan)), WaxWacker 350 (Wacker Chemie AG, Germany), which ispolydimethylsiloxane-polycaprolactone-polydimethylsiloxane ABAtriblockblock copolymer. The toughening agent may be present in thesecond composition in a concentration from about 2% to about 10% basedon the total weight of the second composition.

Embodiments of the invention may be carried into practice by variousways and some illustrative embodiments will be described in thefollowing examples.

EXAMPLES

Exemplary first compositions, i.e. free-radical compositions andexemplary second compositions, i.e. cationic compositions were prepared.The compositions were deposited in various print patterns andcomposition ratios by an Objet Connex500™ 3D printing system ofStratasys Ltd. to form Izod-type test specimens (ASTM D 256-06).Following post treatment (for example, heating), various mechanical andthermo-mechanical properties of the test specimens were tested.

Listed below in Table 1 are different exemplary chemical components andtheir respective trade names, for the free radical (first) composition.

TABLE 1 Components of Free Radical (First) Composition Function in theTrade Name Chemical Type formulation Supplier Photomer 6010 UrethaneAcrylate Oligomer Cognis Oligomer SR-339 Phenoxy ethyl Acrylate MonomerSartomer SR-351 Trimethylol propane Cross-linker Sartomer triacrylateUVI-6976 Mixed Triarylsulfonium Cationic photo- SynasiaHexafluoroantimonate initiator Salts Photomer 4028F Bis Phenol AEthoxylated Acrylic oligomer Cognis Diacrylate SR506D Isobornyl acrylateAcrylic Monomer Sartomer SR833S Tricyclodecane dimethanol AcrylicMonomer Sartomer diacrylate CHVE 1,4-cyclohexane Vinyl Ether ISPdimethanol divinyl ether Monomer V-CAP Vinylcaprolactam Monomer ISPEbecryl 350 Silicon acrylated oligomer Phase separation UCB Chemicalspromoter Trimethylolpropane Sulfur-containing Crosslinker Bruno Bocktri(3- compound Chemische Fabrik mercaptopropionate) HMBH & CO. Uvacure1600 P-(octyloxyphenyl) Cationic Cytec phenyliodonium photoinitiatorhexafluoroantimonate Irgacure I-651 Alpha, alpha-dimethoxy Radical CIBAalpha phenylacetophenone photoinitiator TPO Diphenyl (2,4,6 Radical BASFtrimethylbenzoyl) photoinitiator phosphine oxide BR 970 Urethanediacrylate Acrylic oligomer IGM Speedcure ITX 2-isopropylthioxanthoneCationic Lambson and 4- photosensitizer isopropylthioxanthone BYK 3570Acrylfunctional polyester Additive BYK modified polydimethlsiloxaneCurcumin 1,6-Heptadiene-3,5-dione, Cationic Axowin 1,7-bis(4-hydroxy-3-photosensitizer methoxyphenyl)- DBS-C21 Carbinol Toughener Gelesthydroxyterminated PDMS

Non-limiting examples of possible free radical polymerizablecompositions according to embodiments of the invention are shown inTable 2.

TABLE 2 Free Radical (First) Compositions Component (Trade name)Function A B C D E F G Photomer 4028 Bisphenol A X X X X X X ethoxylatediacrylate SR 339 Phenoxyacrylate X X X X X SR 833S Tricyclodecane X X XX X X X dimethanol diacrylate TPO Radical photoinitiator X X UVI 6976Cationic photoinitiator X (sulfonium salt) Uvacure 1600 Cationicphotoinitiator X X X X X (iodonium salt) Irgacure 651 Radicalphotoinitiator X X X X X BR 970 Aliphatic polyester X urethane acrylateSR 506D Isobornyl acrylate X X DBS-C21 Carbinol X hydroxyterminated PDMS

Listed below in Table 3 are different exemplary chemical components andtheir respective trade names, for the cationic (second) composition.

TABLE 3 Components of Cationic (Second) Composition Function in theTrade Name Chemical Type formulation Supplier Photomer-6010 UrethaneAcrylate Oligomer Oligomer Cognis SR-339 Phenoxy ethyl Acrylate MonomerSartomer SR-351 Trimethylol propane triacrylate Cross-linker SartomerSR506D Isobornyl acrylate Acrylic Monomer Sartomer SR833S Tricyclodecanedimethanol Acrylic Monomer Sartomer diacrylate Uva Cure 1500 3,4Epoxycyclohexylmethyl- Epoxy oligomer Cytec3,4-epoxycyclohexylcarboxylate CHVE 1,4-cyclohexane dimethanol VinylEther Monomer ISP divinyl ether Irgacure I-651 Alpha, alpha-dimethoxyalpha Radical photoinitiator CIBA phenylacetophenone TPO Diphenyl (2,4,6trimethylbenzoyl) Radical photoinitiator BASF phosphine oxide Photomer4028F Bisphenol A ethoxylate diacrylate Acrylic oligomer Cognis PC 1000Difunctional silicon-containing Cationic monomer Polyset epoxy resinSpeedcure ITX 2-isopropylthioxanthone and 4- Cationic Lambsonisopropylthioxanthone photosensitizer BYK 3570 Acrylfunctional polyesterAdditive BYK modified polydimethlsiloxane Curcumin1,6-Heptadiene-3,5-dione, 1,7- Cationic Axowinbis(4-hydroxy-3-methoxyphenyl)- photosensitizer Celloxide 3000 Limonenedioxide Cationic monomer Daicel PC 2000 Difunctional silicon-containingCationic oligomer Polyset epoxy resin Anthracene PhotosensitizerPhotosensitizer Sigma-Aldrich

Non-limiting examples of possible cationic compositions according toembodiments of the invention are shown in Table 4.

TABLE 4 Cationic (Second) Compositions Component (Trade name) Function AB C D E F G Uvacure 1500 Cycloaliphatic epoxy X X X X X X X resinSpeedcure ITX Photosensitizer X X X X Irgacure 651 Radicalphotoinitiator X X X X X Celloxide 3000 Limonene dioxide X PC 2000Difunctional silicon- X containing epoxy resin Curcumin Photo sensitizerX PC 1000 Difunctional silicon- X X containing epoxy resin AnthracenePhotosensitizer X

Exemplary first and second compositions were prepared by puttingweighted amounts of the components in a plastic container and heating toa temperature of 85° C. Then, the components were mixed at roomtemperature using a high shear mixer such as shear mixer available fromCharles Ross & Company, USA until the ingredients were dissolved and ahomogeneous composition was obtained. The final composition was filteredusing a 5-micron filter to remove non-soluble impurities. The viscosityof the composition was determined with a Brookfield DVE type viscometeravailable from Brookfield Company, USA at a temperature of 75° C. for 15minutes using a cylindrical spindle S00 rotating at 30 RPM.

The compositions were jetted using an Objet Connex500™ 3D printingsystem of Stratasys Ltd. in a Digital Material (DM) printing mode toform test samples e.g. Izod-type test specimens (ASTM D 256-06) inseveral ratios and patterns between the first and second composition asdetailed below. The test samples were heated using an oven in either arelatively short process of heating the sample for an hour at 90° C.(short post-printing process) or a longer process that includedgradually heating the samples in an oven for 5 hours starting from 80°C. until 120° C. at intervals of 30 minutes followed by heating thesample at a temperature of 120° C. for another hour and then graduallycooling to a temperature of 60° C. (long post-printing process).

The thermo-mechanical properties of the test samples of the polymerizedcombined compositions were determined by measuring the impact strengthand Heat deflection temperature according to respective ASTM D0256-06and ASTM 648-06 procedures. The impact strength of the specimens wasmeasured by a Resil 5.5 J type instrument (CEAST Series, INSTRON, USAusing an Izod impact test (notched Izod) according to the ASTMInternational organization D-256 standard. The Heat deflectiontemperature (HDT) of the samples was determined according to the ASTMInternational organization D-648 standard. The specimens were testedusing a HDT 3 VICAT instrument (CEAST Series, INSTRON, USA). GlassTransition Temperature (Tg) was determined by a DMA Q800 measurementdevice (TA Instruments (Belgium)). Modulus, strength and elongation wereobtained using Lloyd LR 5k instruments (Lloyd Instruments, UK).

In the following examples, component designations are in weightpercentages. The radical compositions are acrylate-based compositionsand the cationic compositions are epoxy-based compositions. Thecomponents of each of the compositions were mixed to produce homogenousliquid compositions.

Example 1

Radical/acrylic Cationic/epoxy Trade name Function compositioncomposition Photomer Bisphenol A 48 4028F ethoxylate diacrylate SR 339Phenoxyacrylate 10 SR 833S Tricyclodecane 38 dimethanol diacrylateUvacure 1600 Cationic 3 photoinitiator (Iodonium salt) Darocur TPORadical photoinitiator 1 Uvacure 1500 Epoxy-based 99 compound SpeedcureITX Photosensitizer 1 Total 100 100

In Example 1, the radical composition includes both a radicalphotoinitator and a cationic photoinitiator, and the cationiccomposition does not include any photoinitiators.

Test samples were prepared by printing the compositions in a ratio of50%:50% followed by the short post-printing heating process.

The following properties were measured:

HDT 128.2 ± 0.4° C. Flexural strength 112 ± 3 MPa Flexural modulus 3030± 50 Mpa Impact 13 J/m²

Additional test samples were prepared by printing the compositions in aratio of 40%:60% (free-radical composition: cationic composition)followed by the long post-printing heating process.

Example 2 Radical/ Cationic/ acrylic epoxy Trade name Functioncomposition composition Photomer Bisphenol A ethoxylate 46 4028Fdiacrylate SR 339 Phenoxyacrylate 9 SR 833S Tricyclodecane dimethanol 37diacrylate UVI 6976 Cationic photoinitiator 8 (Sulfonium salt) DarocurTPO Radical photoinitiator 1 Uvacure 1500 Epoxy-based compound 99 Total100 100

Test samples in three different patterns of a 50%:50% printing mode wereprepared followed by the short post-printing treatment.

In the first sample, each layer included a 1:1 ratio in a “Chess board”homogenous pattern namely, 50% (by dots) of the layer was occupied bythe acrylic composition and 50% of the layer was occupied by the epoxycomposition. The combined polymerized material exhibits a “curling”tendency in the range of 1-2 mm and a Heat deflection Temperature of 88°C.

In the second sample, the compositions were printed in rows, e.g. onepixel wide, perpendicular to the scan direction. In this comparativeexample, deposition of the same two compositions in the same ratio(50:50 w/w) but in a different pattern results in a “curling” tendencyof 22 mm, which is significantly higher than the “curling” produced bythe “Chess Board” deposition pattern (1-2 mm) although the HDT remainssubstantially similar at 87° C.

In the third sample, the compositions were printed in rows, e.g. onepixel wide, parallel to the scan direction. In this example, the curlingwas measured at 20 mm and the HDT was 84° C.

Additional examples are given below marked as Example 3 and 4.

Example 3

Cationic/ Radical/acrylic epoxy Trade name Function compositioncomposition Photomer Bisphenol A ethoxylate 45 4028F diacrylate SR 339Phenoxyacrylate 9 SR 833S Tricyclodecane 36 dimethanol diacrylateDarocure TPO Radical photoinitiator 2 UVI 6976 Cationic photoinitiator 8(Sulfonium salt) Uvacure 1500 Epoxy-based compound 100

Example 4

Cationic/ Radical/acrylic epoxy Trade name Function compositioncomposition BR 970 Aliphatic polyester 14 urethane acrylate SR 9036Etoxylated (30) 20 bispehnol A dimethacrylate AGI 1030 Difunctionalacrylic 8.61 ester SR 506D Isobornyl acrylate 50 Uvacure 1600 Cationicphotoinitiator 2.77 (Iodonium salt) BYK 3570 Surfactant 2.77 Irgacure651 Radical photoinitiator 1.85 1.9 Uvacure 1500 Epoxy-based compound 93PC 1000 Difunctional silicon- 4.8 containing epoxy resin Speedcure ITXPhotosensitizer 0.3 Total 100 100

Test samples were prepared by printing the composition combinations ofeach of examples 3 and 4 in a ratio of 50%:50% (free-radicalcomposition: cationic composition) followed by the long post-printingheating process. These samples exhibit the following HDT AND Izod impactvalues.

Property Example 3 Example 4 Izod impact <10 20-30 HDT 70 ± 1 C. 77.9 ±0.9 C.

Various aspects of specific embodiments discussed herein may be combinedwith aspects from other embodiments. It will be appreciated by personsskilled in the art that the present invention is not limited by what hasbeen particularly shown and described herein above. Rather the scope ofthe invention is defined by the claims that follow:

1. A printing material kit for inkjet printing comprising: a firstcontainer storing a first composition, the first composition comprisingone or more free-radical polymerizable compounds and a cationicphotoinitiator, wherein the first composition is devoid of compoundsable to undergo cationic photopolymerization within the firstcomposition, and a second container storing a second composition, thesecond composition comprising one or more cationic polymerizablecompounds and is devoid of cationic photoinitiators; wherein at leastone of the first and the second compositions comprises a radicalphotoinitiator.
 2. The printing material kit of claim 1, wherein thefirst composition comprises at least one acrylic monofunctional monomerat concentration of at least 20% by weight relative to the weight of thefirst composition and at least one acrylic oligomer at a concentrationof at least 20% by weight relative to the weight of the firstcomposition.
 3. The printing material kit of claim 1, wherein the secondcomposition comprises a di-functional silicon containing resin.
 4. Theprinting material kit of claim 1, wherein the second composition furthercomprises a cationic photo-sensitizer.
 5. The printing material kit ofclaim 1, wherein the second composition comprises acrylic polymerizablecompounds.
 6. The printing material kit of claim 1, wherein the secondcomposition comprises 3,4-epoxy cyclohexyl methyl-3,4 epoxy cyclohexylcarboxylate at a concentration of at least 50% by weight relative to theweight of the second composition.
 7. The printing material kit of claim6, wherein the 3,4-epoxy cyclohexyl methyl-3,4 epoxy cyclohexylcarboxylate constitutes above 90% by weight of the second composition.8. The printing material kit of claim 1, wherein the cationicphotoinitiator comprises aryliodonium hexafluoroantimonate.
 9. Theprinting material kit of claim 1, wherein the one or more cationicpolymerizable compound is selected from cycloaliphatic epoxides, vinylethers, cyclic sulphides, lactones and siloxanes.
 10. The printingmaterial kit of claim 1, wherein the first composition further comprisesone or more hydroxyl containing compounds, which do not polymerize inthe absence of a cationic photopolymerizable compound.
 11. A method forpreparing a three-dimensional object, comprising a multiplicity ofpolymerized deposited layers, the method comprising: selectivelydepositing by inkjet printing, layer by layer, a first composition and asecond composition onto a receiving media from different dispensers toform polymerizable deposited layers, wherein the first compositioncomprises one or more free-radical polymerizable compounds and acationic photoinitiator and is devoid of compounds able to undergocationic photopolymerization within the first composition, wherein thesecond composition comprises one or more cationic polymerizablecompounds and is devoid of cationic photoinitiators, and wherein atleast one of the first and the second compositions comprises a radicalphotoinitiator; and exposing the polymerizable deposited layers toactinic radiation to initiate, by the radical photoinitiator,polymerization of the one or more free-radical polymerizable compoundsreceived from the first composition, and to initiate, by the cationicphotoinitiator, polymerization of the one or more cationic polymerizablecompounds received from the second composition.
 12. The method of claim11, wherein the first and second compositions are dispensed so as toform a substantially isotropic distribution within each of thepolymerizable deposited layers.
 13. The method of claim 11, wherein thefirst and second compositions are dispensed such that in each of thepolymerizable deposited layers, a ratio between deposited droplets ofthe first composition and deposited droplets of the second compositionis between about 25% first composition to about 75% second compositionand about 75% first composition to about 25% second composition.
 14. Themethod of claim 13, wherein the ratio in at least one layer of thepolymerizable deposited layers is different from the ratio in at leastone other of the polymerizable deposited layers.
 15. The method of claim11, wherein the first composition comprises at least one acrylicmonofunctional monomer at concentration of at least 20% by weightrelative to the weight of the first composition and at least one acrylicoligomer at a concentration of at least 20% by weight relative to theweight of the first composition.
 16. The method of claim 11, wherein thesecond composition comprises at least one of: a di-functional siliconcontaining resin, a cationic photo-sensitizer and acrylic polymerizablecompounds.
 17. The method of claim 11, wherein the second compositioncomprises 3,4-epoxy cyclohexyl methyl-3,4 epoxy cyclohexyl carboxylateat a concentration of at least 50% by weight relative to the weight ofthe second composition.
 18. The method of claim 11, wherein the cationicphotoinitiator comprises aryliodonium hexafluoroantimonate.
 19. Themethod of claim 11, wherein the one or more cationic polymerizablecompound is selected from cycloaliphatic epoxides, vinyl ethers, cyclicsulphides, lactones and siloxanes.
 20. The method of claim 11, whereinthe first composition further comprises one or more hydroxyl containingcompounds, which do not polymerize in the absence of a cationicphotopolymerizable compound.
 21. A three-dimensional object comprising amultiplicity of polymerized deposited layers formed by the method ofclaim 11.